Commuting: Sorrento-CinqueTerre Ticket Booking in Advance?

Sorrento to Cinque Terre is feasible, but plan on at least 8 hours of train travel that day. For example, there's an IC Plus train that departs Naples at 9:24 and arrives in La Spezia 15:51 with no changes. You would need to depart Sorrento between 7:30-8:00 to catch the train in Naples. Advance purchase for the Circumvesuviana isn't needed. For your Naples to La Spezia ticket, buy it at a travel agency in Sorrento a day or 2 in advance. English is widely spoken in Sorrento and the travel agent can help you sort through the options.

Thanks a lot Frank!

Also, do we need to book any of the other trains in advance?- All my dates are fixed and so I don't want to run into any problems of not getting ticket availability.

If I can book in advance then what is a good website to book these train tickets and what is the Amica fare?

I always buy train tickets day of departure or a few days in advance and have never had a problem. You can purchase tickets on the Trenitlia website up to 60 days before travel date, but I find it easier to buy them in Italy. One advantage of buying in advance is that you may be able to get discounted tickets for the high speed trains. Search for past posts on that topic - Norm's are very good. The Amica fare is a 20% discount available until midnight day before travel or until sold out. Most trains between Naples, Rome and Florence require reservations, but there are at least 25 trains per day (probably more) so you're unlikely to get shut out. Some of the trains you plan to take are regional or local trains. You can always get on those, but you cannot reserve a seat. Italian trains are timed to connect and are pretty reliable in that regard, so don't be too concerned about missing connentions. Even if you do, there's usually another train to your destination within the hour.